Piezoelectric material, piezoelectric element, liquid discharge head, ultrasonic motor, and dust cleaning device

ABSTRACT

A piezoelectric material including a strontium calcium sodium niobate-based tungsten bronze structure metal oxide having a high degree of orientation is provided. A piezoelectric element, a liquid discharge head, an ultrasonic motor, and a dust cleaning device including the piezoelectric material are also provided. A piezoelectric material includes a tungsten bronze structure metal oxide that includes metal elements which are strontium, calcium, sodium, and niobium, and tungsten. The metal elements satisfy following conditions on a molar basis: when Sr/Nb=a, 0.320≦a≦0.430, when Ca/Nb=b, 0.008≦b≦0.086, and when Na/Nb=c, 0.180≦c≦0.200. The tungsten content on a metal basis is 0.40 to 3.20 parts by weight relative to 100 parts by weight of the tungsten bronze structure metal oxide. The tungsten bronze structure metal oxide has a c-axis orientation.

TECHNICAL FIELD

The present invention relates to piezoelectric materials. In particular, it relates to a lead-free piezoelectric material. It also relates to a piezoelectric element, a liquid discharge head, an ultrasonic motor, and a dust cleaning device that use the lead-free piezoelectric material.

BACKGROUND ART

Lead zirconate titanate (referred to as “PZT” hereinafter) that contains lead is a piezoelectric material most widely used in various piezoelectric devices. Attempts have been made to replace lead-containing piezoelectric materials by lead-free piezoelectric materials. This is to address the concern that lead-containing piezoelectric devices once discarded and exposed to acid rain may inflict damage on ecosystem as the lead component in the piezoelectric materials may leach into earth. Thus, various proposals of piezoelectric material have been made.

One example of the lead-free piezoelectric material is a strontium calcium sodium niobate (SCNN) having a tungsten bronze structure. PTL 1 discloses a technique of improving the piezoelectric property by adding a rare earth element to SCNN. PTL 2 discloses a technique of improving the piezoelectric property by replacing Na sites with K. However, since the unit cell of crystals having a tungsten bronze structure has a high shape anisotropy, the polar axis direction lies only in the c-axis direction, i.e., the short side direction. Accordingly, there have been fewer effective domains that can contribute to piezoelectricity and the piezoelectric properties have been unsatisfactory.

NPL 1 discloses a technique of improving the characteristics of SCNN by orienting SCNN by using a magnetic field (magnetic orientation). Magnetic orientation can increase the number of effective magnetic domains that contribute to piezoelectricity. NPL 2 discloses an example of fabricating a prototype ultrasonic motor by using magnetically oriented SCNN. However, this ultrasonic motor requires application of a voltage about 10 times greater than that required by a PZT ultrasonic motor and has unsatisfactory characteristics.

The present invention provides a lead-free strontium calcium sodium niobate-based tungsten bronze structure metal oxide having a high degree of orientation and high piezoelectric property. A piezoelectric element, a liquid discharge head, an ultrasonic motor, and a dust cleaning device that use the piezoelectric material are also provided.

CITATION LIST Patent Literature

-   PTL 1 Japanese Patent Laid-Open No. 2000-169229 -   PTL 2 Japanese Patent Laid-Open No. 2003-261379

Non Patent Literature

-   NPL 1 Japanese Journal of Applied Physics, 2008, vol. 47, No. 9, pp.     7693-7697 -   NPL 2 Japanese Journal of Applied Physics, 2008, vol. 47, No. 5, pp.     4242-4247

SUMMARY OF INVENTION

The present invention provides a piezoelectric material that includes a tungsten bronze structure metal oxide that includes metal elements which are strontium, calcium, sodium, and niobium; and tungsten. The metal elements satisfy following conditions on a molar basis: when Sr/Nb=a, 0.320≦a≦0.430, when Ca/Nb=b, 0.008≦b≦0.086, and when Na/Nb=c, 0.180≦c≦0.200. A tungsten content on a metal basis is 0.40 to 3.20 parts by weight relative to 100 parts by weight of the tungsten bronze structure metal oxide. The tungsten bronze structure metal oxide has a c-axis orientation.

The present invention also provides a piezoelectric element that includes a first electrode, a piezoelectric material, and a second electrode. The present invention also provides a liquid discharge head, an ultrasonic motor, and a dust cleaning device that include this piezoelectric element.

The present invention can provide a piezoelectric material including a strontium calcium sodium niobate-based tungsten bronze structure metal oxide having a high degree of orientation. Since the piezoelectric material is lead-free, the impact on the ecosystem is low. A piezoelectric element, a liquid discharge head, an ultrasonic motor, and a dust cleaning device that have high durability can also be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are diffraction patterns of piezoelectric materials taken by X-ray diffraction analysis in a 2θ-θ mode.

FIG. 2 is a schematic view of a piezoelectric element according to an embodiment.

FIG. 3A and FIG. 3B are schematic diagrams showing one example of a liquid discharge head according to an embodiment.

FIG. 4A is a schematic diagram showing an ultrasonic motor according to an embodiment.

FIG. 4B is a schematic diagram showing an ultrasonic motor according to another embodiment.

FIGS. 5A and 5B are schematic diagrams showing a dust cleaning device according to an embodiment.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will now be described.

A piezoelectric material according to one embodiment of the invention includes a tungsten bronze structure metal oxide that contains metal elements, Sr, Ca, Na, and Nb that satisfy conditions below on a molar basis, and W:

when Sr/Nb=a, 0.320≦a≦0.430,

when Ca/Nb=b, 0.008≦b≦0.086, and

when Na/Nb=c, 0.180≦c≦0.200.

The tungsten (W) content on a metal basis is 0.40 to 3.20 parts by weight relative to 100 parts by weight of the tungsten bronze structure metal oxide. The tungsten bronze structure metal oxide has a c-axis orientation.

In the present invention, the term “tungsten bronze structure” refers to what is generally known as tetragonal tungsten bronze (TTB) structure, and not H_(x)WO₃ (tungsten bronze) or a hexagonal tungsten bronze (HTB) structure known for its electrochromic phenomena.

The piezoelectric material has a tungsten bronze structure and contains Sr, Ca, Na, and Nb. It is considered that Sr, Ca, and Na occupy A sites and Nb occupies B sites. When a tungsten bronze structure is constituted by Sr, Ca, Na, and Nb, a high mechanical quality factor and high piezoelectricity can be achieved. That the piezoelectric material has a tungsten bronze structure can be confirmed by, for example, X-ray diffraction analysis. A diffraction pattern of a tungsten bronze structure measured in a 2θ-θ mode has a peak attributable to (001) plane near 2θ=22.7°, a peak attributable to (211) plane near 2θ=32.5°, and a peak attributable to (002) plane near 2θ=46.5°.

The phrase “on a molar basis” means that the amounts of the elements, such as Sr, Ca, Na, and Nb, contained in the tungsten bronze structure determined by X-ray fluorescence analysis, inductively coupled plasma (ICP) atomic emission spectroscopy, atomic absorption spectrometry, or the like are converted to ratios in terms of moles.

The ratio a of Sr to Nb on a molar basis is 0.320≦a≦0.430. When the ratio a is less than 0.320, the piezoelectric property may be low. When the ratio a is greater than 0.430, the amount of heat generated during operation of the device increases and the characteristics may be degraded.

The ratio b of Ca to Nb on a molar basis is 0.008≦b≦0.086. When the ratio b is less than 0.008, the amount of heat generated during operation of the device increases and the characteristics may be degraded. When the ratio b is greater than 0.086, the piezoelectric property may be low.

The ratio c of Na to Nb on a molar basis is 0.180≦c≦0.200. When the ratio c is less than 0.180, piezoelectric property may be low. When the ratio c is greater than 0.200, the mechanical quality factor may be low and the durability may be degraded.

The W content “on a metal basis” is determined by measuring the contents of metals, such as Ba, Bi, Nb, and W from the tungsten bronze structure metal oxide by X-ray fluorescence analysis, ICP atomic emission spectroscopy, atomic absorption spectrometry, or the like, converting the measured contents of the elements, such as Sr, Ca, Na, and Nb, constituting the tungsten bronze structure into oxide-based values, and determining the ratio of the weight of tungsten (W) relative to the total of the converted metal contents.

Three axes that extend along the sides of the unit cell constituting a crystal are referred to as a-axis, b-axis, and c-axis. In general, a tungsten bronze structure has a cuboidal unit cell and the three axes intersect one another perpendicularly. The length of each axis is referred to as “axis length”. The c-axis length of the tungsten bronze structure is as short as about one third of the a-axis length or the b-axis length. In this specification, the term “(001) plane” refers to a plane to which the c-axis is normal.

In this specification, “orient” means that all or part of subject crystal faces are oriented in a particular direction. The “degree of orientation” indicates the degree at which the crystal faces are oriented. The degree of orientation increases when there are more portions where subject crystals faces are oriented in a particular direction. The term “c-axis orientation” means that crystal faces that lie in the c-axis direction are oriented, i.e., (001) planes are oriented. In other words, the “c-axis orientation” and the “(001) plane orientation” have the same meaning. The pole axis direction, which is the function-occurring axis of a tungsten bronze structure as a piezoelectric material, is the c-axis direction. The piezoelectric property of a tungsten bronze structure metal oxide is enhanced when c-axis orientation is achieved.

In general, a tungsten bronze structure metal oxide is expressed by a chemical formula, A₄₋₆B₁₀O₃₀. In the chemical formula, A indicates elements that occupy A sites. A1 sites (12-coordinated box-shaped sites when viewed in the c-axis direction) and A2 sites (15-coordinated pentagonal sites when viewed in the c-axis direction) are not distinguished. Sr, Ca, and Na primarily occupy one of two specific positions called A1 sites and A2 sites that exist around the oxygen octahedron. The maximum value of the sum of the numbers of A1 sites and A2 sites is 6.

In the chemical formula above, B indicates elements that occupy B sites. Pentavalent elements primarily occupy the B sites. In this invention, B is primarily Nb and exists inside the oxygen octahedron.

In the present invention, tungsten (W) sometimes occupies some of the B sites but this is not essential. The form in which W is contained may be an oxide, a metal, or a metal ion.

When W is contained in the piezoelectric material, the dielectric constant of the tungsten bronze structure metal oxide increases and the piezoelectric constant increases along with this. Incorporation of W also enhances the anisotropic energy of magnetic susceptibility and improves the degree of orientation when a magnetic field is applied. This is presumably due to the following reason. That is, when W replaces some of the Nb atoms in the B sites, the magnetic susceptibility or the magnetic moment in all directions of the unit cell undergo change. As a result, the anisotropy is enhanced by the difference in magnetic susceptibility among different directions of the unit cell. This phenomenon is presumably caused by the fact that the ionic radius (0.062 nm) of a hexavalent W ion is close to the ionic radius (0.069 nm) of Nb. In other words, incorporation of W brings two effects: increasing the dielectric constant and improving the magnetic susceptibility. Since the piezoelectric material has a high degree of c-axis orientation, the piezoelectric constant thereof increases significantly.

The W content on a metal basis is 0.40 to 3.20 parts by weight relative to 100 parts by weight of the tungsten bronze structure metal oxide. At a W content more than 3.20 parts by weight, the curie temperature may drop significantly. If the curie temperature is low, polarization treatment becomes difficult and the inherent piezoelectric properties of the piezoelectric material may not be fully exhibited. At a W content less than 0.40 parts by weight, the properties may not change much compared to when no W is contained. The W content is preferably 0.40 to 0.29 parts by weight and more preferably 1.00 to 2.90 parts by weight.

The Lotgering factor F of the piezoelectric material may be 0.81 to 1.00. A Lotgering factor is an indicator of the degree of c-axis (or (001)) orientation of the tungsten bronze structure metal oxide determined by X-ray diffraction analysis.

Although there are several indicators of the degree of orientation, a Lotgering factor F is used in this specification to indicate the degree of orientation. A Lotgering factor F larger than 0 indicates that the subject crystal faces are oriented.

When the Lotgering factor F is less than 0.81, there are fewer effective domains that can contribute to piezoelectricity and the piezoelectric properties are not sufficient. As the Lotgering factor F approaches 1, more effective domains that can contribute to piezoelectricity are generated and thus the piezoelectric constant is increased. When the Lotgering factor F is 1, the only peak detected is the diffraction peak attributable to the subject crystal plane. In other words, all crystals detectable by X-ray diffraction analysis are aligned and oriented in the subject direction.

The Lotgering factor F is calculated by 2θ-θ X-ray diffraction analysis. It is calculated using equation 1 below by using the cumulative peak intensity of X rays diffracted by the target crystal plane at 2θ in the range of 10° to 70°:

F=(ρ−ρ₀)/(1−ρ₀)  (Equation 1)

In equation 1, in the case of c-axis orientation, ρ₀ is determined by using the X-ray diffraction intensity (I₀) of a randomly oriented sample and calculated from equation (2) below as the ratio of the total of the diffraction intensities of the (001) plane (all faces perpendicular to the c-axis, 1=1 or 2) to the total diffraction intensity:

ρ₀ =ΣI ₀(001)/ΣI ₀(hkl)  (Equation 2)

In equation 1, in case of the c-axis orientation, ρ is determined by using the X-ray diffraction intensity (I) of an oriented sample and calculated from equation (3) below as the ratio of the total of the diffraction intensities of the (001) plane to the total diffraction intensity as with equation (2) above:

ρ=ΣI(001)/ΣI(hkl)  (Equation 3)

The tungsten bronze structure metal oxide in the piezoelectric material may be represented by (Sr_(1-x)Ca_(x))_(y)Na₂Nb₁₀O₃₀ (0.020≦x≦0.200 and 4.0≦y≦4.3).

The molar ratio x of Ca occupying the A sites is 0.020≦x≦0.200. When the molar ratio x is less than 0.020, the amount of heat generated during operation of the device increases and the characteristics may be degraded. When the molar ratio x is greater than 0.200, abnormal grain growth is likely to occur during sintering. A sintered material that has undergone abnormal grain growth is susceptible to cracking and breaking, has a low fracture strength, and thus is not suitable for use in devices.

The sum of the molar ratios of Sr and Ca occupying the A sites is denoted by y and y is 4.0≦y≦4.3. When y is less than 4.0, the A sites are deficient relative to the B sites. When A sites are deficient, defective sites occurs and form oxygen defects. Thus, the insulating property and the piezoelectric property may be adversely affected. When y is greater than 4.3, the A sites are in excess, and this results in charge imbalance. Charge imbalance may significantly degrade the insulating property.

The piezoelectric material may contain Mn in addition. The Mn content on a metal basis may be 0.10 to 0.90 parts by weight relative to 100 parts by weight of the tungsten bronze structure metal oxide.

Since the piezoelectric material contains Mn, the mechanical quality factor can be improved without degrading the Lotgering factor F. The Mn content is 0.10 to 0.90 parts by weight. When the Mn content is less than 0.10 parts by weight, the properties may not change much compared to when no Mn is contained. When the Mn content exceeds 0.90 parts by weight, the piezoelectric properties may deteriorate and non-tungsten bronze structure phases may occur. The Mn content may be 0.10 to 0.60 parts by weight. The mechanical quality factor (Qm) is a factor that indicates elastic loss caused by vibration when the piezoelectric material is evaluated as a vibrator. When the magnitude of the mechanical quality factor is electrically measured, the mechanical quality factor is observed as the sharpness of a resonance curve. In other words, the mechanical quality factor is a factor that indicates the sharpness of the resonance of a vibrator.

The region of the piezoelectric material in which Mn exists is not limited. Manganese may occupy the sites or lie in grain boundaries without occupying the sites. The form of incorporation of Mn may be any of metal, oxide, and metal ion. When Mn is incorporated in form of metal ion, the valence may be 2, 3, 4, or 6.

Ba, Mg, K, and Li may occupy the A sites of the tungsten bronze structure metal oxide and elements other than Nb may occupy the B sites of the tungsten bronze structure metal oxide to facilitate production of the piezoelectric material or adjust the physical properties of the piezoelectric material. Examples of elements to be contained in the B sites include not only pentavalent metal elements such as Ta and V but also trivalent and tetravalent metal elements, more specifically, Fe, Al, Ti, and Zr. The content of these elements to be contained in the B sites is preferably 20 mol % or less and more preferably 5 mol % or less of all B-site elements. The decrease in total valence of the B-site elements may be compensated by increasing the amount of the A-site elements.

Elements such as Cu, Zn, and Co may be added to the piezoelectric material to facilitate production of the piezoelectric material and adjust the physical properties of the piezoelectric material. The amount of such additional element may be 5 parts by weight or less relative to 100 parts by weight of the tungsten bronze structure metal oxide. When more than 5 parts by weight of such elements are added, structures other than the tungsten bronze structure may occur and the insulating property may decrease.

The metal elements that occupy the respective sites of the piezoelectric material can be identified by, for example, a Rietveld method. According to the Rietveld method, not only the type of metal that occupies the sites but also the ratio thereof can be identified.

The method for preparing the piezoelectric material is not particularly limited. The piezoelectric material may be prepared by a common method of sintering a powder obtained by using raw materials, such as oxides, nitrates, and oxalates of the metals constituting the piezoelectric material or a shaped body prepared by slip casting, the sintering being performed under a normal pressure. Examples of other techniques that can be employed include electric heating, microwave sintering, millimeter wave sintering, and hot isostatic pressing. From the viewpoint of suitability to magnetic orientation, a technique of sintering, under a normal pressure, a shaped body formed by slip-casting a slurry may be employed.

The method for orienting the piezoelectric material is not limited to a particular method. For example, a grain orientation method using a doctor blade or a magnetic orientation method that uses a high magnetic field may be employed. A magnetic orientation method is preferred since a shaped body having a c-axis orientation can be easily obtained. Magnetic orientation that uses a rotating magnetic field is more preferable.

The method for adding W, Mn, etc., to the tungsten bronze structure metal oxide is not particularly limited. For example, W may be added after the elements constituting the tungsten bronze structure metal oxide are mixed, calcined, and reacted. Alternatively, W may be mixed with the elements constituting the tungsten bronze structure metal oxide, and calcined and reacted simultaneously.

FIG. 2 is a schematic view of a piezoelectric element according to an embodiment. The piezoelectric element of this embodiment includes a piezoelectric material, and a first electrode and a second electrode disposed on the piezoelectric material. The first and second electrodes may be disposed on one surface of the piezoelectric material or on opposing surfaces of the piezoelectric material so as to sandwich the piezoelectric material.

FIG. 3A and FIG. 3B are schematic diagrams showing one example of a liquid discharge head according to an embodiment of the present invention. The liquid discharge head includes a piezoelectric element 101. The piezoelectric element 101 includes a first electrode 1011, a piezoelectric material 1012, and a second electrode 1013. The piezoelectric material 1012 is patterned according to need as shown in FIG. 3B.

FIG. 3B is a schematic diagram of the liquid discharge head. The liquid discharge head includes discharge ports 105, individual liquid chambers 102, connecting holes 106 connecting the individual liquid chambers 102 to the discharge ports 105, liquid chamber walls 104, a common liquid chamber 107, a vibrating plate 103, and the piezoelectric element 101. Although the piezoelectric element 101 in the drawing has a rectangular shape, it may have any other shape, such as elliptical, circular, rectangular parallelepiped, etc. In general, the piezoelectric material 1012 has a shape corresponding to the shape of the individual liquid chambers 102.

The piezoelectric element 101 of the liquid discharge head and nearby parts will now be described in detail with reference to FIG. 3A. FIG. 3A is a cross-sectional view of the piezoelectric element taken in the width direction of the liquid discharge head shown in FIG. 3B. Although the cross-sectional shape of the piezoelectric device 101 is rectangular in this embodiment, the shape may be a trapezoidal or reversed trapezoidal shape.

In the drawing, the first electrode 1011 is used as a lower electrode and the second electrode 1013 is used as an upper electrode. However, arrangement of the first electrode 1011 and the second electrode 1013 is not limited to this. The first electrode 1011 may be used as a lower electrode or an upper electrode. The second electrode 1013 may be used as an upper electrode or a lower electrode. A buffer layer 108 may be interposed between the vibrating plate 103 and the lower electrode.

It should be noted here that these names of the components are determined depending on the process of producing the device and the advantageous effects of the present invention can be achieved in any case. According to this liquid discharge head, the vibrating plate 103 moves up and down by expansion and contraction of the piezoelectric material 1012 and pressure the liquid contained in the individual liquid chambers 102 so that the liquid is discharged from the discharge ports 105. The liquid discharge head can be used in printers and for electronic device production.

The thickness of the vibrating plate 103 is 1.0 μm or more and 15 μm or less and preferably 1.5 μm or more and 8 μm or less. The material for the vibrating plate 103 is not limited but may be silicon. Silicon constituting the vibrating plate may be doped with boron or phosphorus. The buffer layer and the electrode layer on the vibrating plate may form part of the vibrating plate.

The thickness of the buffer layer 108 is 5 nm or more and 300 nm or less and more preferably 10 nm or more and 200 nm or less. The size of the discharge ports 105 is 5 μm or more and 40 μm or less in terms of circular equivalent diameter. The shape of the discharge ports 105 may be circular but may be a star shape, a rectangular shape, or a triangular shape.

Ultrasonic motors using the piezoelectric element will now be described. FIGS. 4A and 4B are schematic views showing examples of embodiments of ultrasonic motors. FIG. 4A shows an ultrasonic motor including a piezoelectric element constituted by a single plate. This ultrasonic motor includes a vibrator 201, a rotor 202 abutting the sliding surface of the vibrator 201 under pressure from a pressurizing spring not shown in the drawing, and an output shaft 203 integrally formed with the rotor 202. The vibrator 201 includes a metal elastic ring 2011, a piezoelectric element 2012, and an organic adhesive 2013 (epoxy- or cyanoacrylate-based adhesive, for example) that bonds the piezoelectric element 2012 to the metal elastic ring 2011. The piezoelectric element 2012 includes a first electrode, a second electrode, and a piezoelectric material interposed between the first and second electrodes (not shown).

When two-phase AC voltage (phase difference: π/2) is applied to the piezoelectric element, a bending travelling wave occurs in the vibrator 201, and individual points on the sliding surface of the vibrator 201 undergo elliptical motions. When the rotor 202 is pressure-contacted with the sliding surface of the vibrator 201, the rotor 202 receives friction force from the vibrator 201 and rotates in the direction opposite to the bending travelling wave. An object to be driven not shown in the drawing is joined to the output shaft 203 and is driven by the turning force of the rotor 202. When a voltage is applied to the piezoelectric material, the piezoelectric material expands and contracts by a transversal piezoelectric effect. When an elastic body, such as a metal, is in contact with the piezoelectric element, the elastic body is bent by the expansion and contraction of the piezoelectric material. The type of ultrasonic motor described here utilizes this principle.

FIG. 4B shows an example of an ultrasonic motor that includes a piezoelectric element having a multilayer structure. A vibrator 204 includes cylindrical metal elastic bodies 2041 and a multilayer piezoelectric element 2042 interposed between the cylindrical metal elastic bodies 2041. The multilayer piezoelectric element 2042 is an element that includes two or more layers of piezoelectric materials not shown in the drawing and includes a first electrode and a second electrode on the outer surface of the multilayer structure and inner electrodes inside the multilayer structure. The cylindrical metal elastic bodies 2041 are joined with bolts to fix the multilayer piezoelectric element 2042 therebetween to make a vibrator 204.

When alternating current voltages having phases different from each other are applied to the multilayer piezoelectric element 2042, the vibrator 204 excites two vibrations orthogonal to each other. These two vibrations are combined to form circular vibrations for driving at the tip of the vibrator 204. Note that an annular groove is formed in the upper portion of the vibrator 204 to increase the displacement of the vibration for driving.

A rotor 205 pressure-contacts the vibrator 204 due to a spring 206 for pressurization and obtains frictional force for driving. The rotor 205 is rotatably supported by bearings.

Next, a dust cleaning device using a piezoelectric element according to an embodiment of the present invention is described.

FIGS. 5A and 5B are schematic diagrams showing an embodiment of a dust cleaning device. A dust cleaning device 310 includes a plate-shaped piezoelectric element 330 and a vibrating plate 320. Although the material of the vibrating plate 320 is not limited, a transparent material or a light-reflecting material may be used in the vibrating plate 320 when the dust cleaning device 310 is to be used in optical devices.

The piezoelectric element may be used in a liquid discharge head, an ultrasonic motor, etc., as mentioned above. When the lead-free piezoelectric material containing a tungsten bronze structure metal oxide is used in a liquid discharge head, a nozzle density and discharge force comparable or superior to that of a liquid discharge head that uses a lead-containing piezoelectric material can be achieved. When the lead-free piezoelectric material containing a tungsten bronze structure metal oxide is used in an ultrasonic motor, driving force and durability comparable or superior to that of an ultrasonic motor that uses a lead-containing piezoelectric material can be achieved. When the lead-free piezoelectric material containing a tungsten bronze structure metal oxide is used in a dust cleaning device, a dust cleaning efficiency comparable or superior to that of a dust cleaning device that uses a lead-containing piezoelectric material can be achieved.

The piezoelectric material can also be used in devices other than the liquid discharge head and motor, such as ultrasonic vibrators, piezoelectric actuators, piezoelectric sensors, and ferroelectric memories.

EXAMPLES

The piezoelectric material will now be described in specific detail through examples below. These examples do not limit the scope of the present invention.

Examples 1 to 8

Strontium carbonate, calcium carbonate, sodium carbonate, niobium oxide, and tungsten oxide powders were used as the raw materials. The powders were weighed and dry-mixed in a mortar. The mixture was placed in an alumina crucible and calcined in air at 1000° C. to 1200° C. for 2 to 5 hours.

A slurry was prepared from the calcined powder. That is, the calcined powder was pulverized, mixed with particular amounts of pure water and a dispersant, and dispersed with a pot mill to obtain a slurry.

A magnetic field was then applied to the slurry to conduct magnetic-field processing. Magnetic field processing was conducted using a superconducting magnetic (JMTD-10T180, produced by Japan Superconductor Technology, Inc.). A 10 T magnetic field was generated by the superconducting magnet, and a table was rotated at 30 rpm in a direction perpendicular to the direction of the magnetic field by using a nonmagnetic ultrasonic motor that can be rotated and driven in magnetic fields. A plaster mold was placed on the table, and the slurry was poured into the plaster mold on the rotating table to conduct slip-cast molding.

The molded body was fired in an electric furnace at 1300° C. to 1400° C. for 6 hours in air. After a surface of the resulting sintered body was cut, structural analysis by X-ray diffraction analysis (XRD) and compositional analysis by X-ray fluorescence analysis were conducted.

FIGS. 1A and 1B are diagrams observed by 2θ-θ XRD. FIG. 1A is the XRD pattern of a piezoelectric material of Example 4 and FIG. 1B is the XRD pattern of a piezoelectric material of Comparative Example 1. The patterns show that both profiles are that of a tungsten bronze structure metal oxide single phase. The term “single phase” is used when all or nearly all parts of the sintered body are constituted by tungsten bronze structure crystals.

In Example 4 in which the magnetic-field processing is conducted, the intensity of the peak attributable to (001) is high, which indicates that c-axis orientation is achieved. The Lotgering factor F was calculated from the XRD results to evaluate the degree of (001) orientation.

The results of the compositional analysis showed that the piezoelectric material had a tungsten bronze structure represented by (Sr_(1-x),Ca_(y))_(y)Na₂Nb₁₀O₃₀.

The sintered body was polished to a thickness of 1 mm and heat-treated in air at 500° C. to 1000° C. for 1 hour to remove organic matter on the surfaces. Au electrodes 500 μm in thickness were formed by DC-sputtering on both surfaces of the sintered body. The resulting product was cut to 2.5 mm×10 mm by using a cutting machine and used for evaluating various electrical characteristics.

Polarizing was conducted by applying a voltage of 2 to 5 kV/cm for 10 minutes at 120° C. to 200° C. in a silicone oil. The piezoelectric property was evaluated using a d₃₃ meter (PiezoMeter System produced by Piezotest).

Table 1 shows the composition, Lotgering factor F, and d₃₃ of the piezoelectric materials. In the table, a denotes the ratio Sr/Nb on a molar basis, b denotes the ratio Ca/Nb on a molar basis, c denotes the ratio Na/Nb on a molar basis, x denotes the molar ratio of Ca occupying the A-sites, and y denotes the sum of molar ratios of Sr and Ca occupying the A-sites. In Table 1, “W parts by weight” indicates the ratio of the weight of metallic tungsten to the total weight of the elements, such as Sr, Ca, Na, and Nb, constituting the tungsten bronze structure on an oxide basis, F denotes the Lotgering factor F indicating the degree of orientation of (001) plane, and d₃₃ denotes the piezoelectric constant d₃₃.

Comparative Example 1

A slurry was prepared as in Examples 1 to 8.

Then a plaster mold was placed on a table, and slip-cast molding was conducted by pouring the slurry into the plaster mold on the rotating table without conducting magnetic-field processing.

Then a sample of Comparative Example 1 was obtained by sintering as in Examples 1 to 8. The electrical characteristics of the sample of Comparative Example 1 were evaluated as in Examples 1 to 8. The composition, F, and d₃₃ of the obtained piezoelectric material are shown in Table 1.

Comparative Examples 2 to 4 and 6 to 8

Molded bodies were obtained as in Examples 1 to 8 including performance of the magnetic-field processing except that the tungsten oxide powder was not used.

Each molded body was sintered as in Examples 1 to 8 to prepare samples of Comparative Examples 2 to 4 and 6 to 8. The electrical characteristics of the samples of Comparative Examples 2 to 4 and 6 to 8 were evaluated as in Examples 1 to 8. The composition, F, and d₃₃ of the obtained piezoelectric materials are shown in Table 1.

Comparative Example 5

A molded body was obtained as in Examples 1 to 8 including performance of the magnetic-field processing.

The molded body was sintered as in Examples 1 to 8 including performance of the magnetic-field processing to obtain a sample of Comparative Example 5. The electrical characteristics of the sample of Comparative Example 5 were evaluated as in Examples 1 to 8. The composition, F, and d₃₃ of the obtained piezoelectric material are shown in Table 1.

TABLE 1 Composition Sr/Nb = Ca/Nb = Na/Nb = W parts by d33 a b c x y weight F [pC/N] Example 1 0.392 0.008 0.200 0.020 4.00 1.60 0.92 102 Example 2 0.320 0.080 0.200 0.200 4.00 1.60 0.93 108 Example 3 0.360 0.040 0.200 0.100 4.00 0.40 0.81 101 Example 4 0.360 0.040 0.200 0.100 4.00 1.60 0.95 147 Example 5 0.360 0.040 0.200 0.100 4.00 2.90 0.97 160 Example 6 0.387 0.043 0.200 0.100 4.30 1.60 0.92 103 Example 7 0.421 0.009 0.200 0.020 4.30 1.60 0.92 100 Example 8 0.344 0.086 0.200 0.200 4.30 1.60 0.90 100 Comparative 0.360 0.040 0.200 0.100 4.00 1.60 0.00 40.8 Example 1 Comparative 0.392 0.008 0.200 0.020 4.00 0.00 0.42 51.1 Example 2 Comparative 0.320 0.080 0.200 0.200 4.00 0.00 0.44 48.7 Example 3 Comparative 0.360 0.040 0.200 0.100 4.00 0.00 0.68 48.7 Example 4 Comparative 0.360 0.040 0.200 0.100 4.00 0.20 0.68 88.2 Example 5 Comparative 0.387 0.043 0.200 0.100 4.30 0.00 0.44 47.3 Example 6 Comparative 0.421 0.009 0.200 0.020 4.30 0.00 0.42 44.6 Example 7 Comparative 0.344 0.086 0.200 0.200 4.30 0.00 0.44 43.7 Example 8

Comparison of Example 4 (with magnetic-field processing) and Comparative Example 1 (without magnetic-field processing) having the same composition and the same W content shows that the piezoelectric constant d₃₃ is significantly increased by conducting the magnetic orientation.

Comparison of Example 1 (with magnetic-field processing) having a W content of 1.60 parts by weight and Comparative Example 2 (with magnetic field processing) containing no W shows that the Lotgering factor F and the piezoelectric constant d₃₃ of the Example 1 are improved. The same tendency was observed in comparison between Example 1 and Comparative Example 2, between Example 2 and Comparative Example 3, between Example 6 and Comparative Example 6, between Example 7 and Comparative Example 7, and between Example 8 and Comparative Example 8. These results suggest that incorporation of W in the material system containing Sr, Ca, Na, and Nb increases the Lotgering factor F and the piezoelectric constant d₃₃ when magnetic-field processing is conducted.

Next, Examples 3 to 5 (with magnetic field processing) and Comparative Examples 4 and 5 (with magnetic field processing) are compared. All samples had the same composition except for the W content. Comparison shows that the Lotgering factor F and the piezoelectric constant d₃₃ increase with the W content. In Comparative Example 5 in which the W content was 0.20 parts by weight, the Lotgering factor F was not significantly different from that of a sample with zero W content. These results suggest that the W content may be 0.40 to 3.20 parts by weight. In a sample having a W content of 3.50 parts by weight, the curie temperature dropped to 100° C. This sample was polarized but the piezoelectric constant d₃₃ was 55 pC/N and thus was not satisfactory. This shows that the curie temperature decreased significantly at a W content greater than 3.20 and the material is not suitable for practical application.

These results also suggest that the range of the molar ratio x of Ca occupying the A-sites may be 0.020≦x≦0.200 and that the range of y, which is the sum of the molar ratios of Sr and Ca occupying the A-sites, may be 4.0≦y≦4.3.

In view of the above, Table 1 shows that incorporation of W in the material system containing Sr, Ca, Na, and Nb increases the Lotgering factor F and the piezoelectric constant d₃₃ when magnetic-field processing is conducted.

Examples 9 to 11

Piezoelectric materials of these examples differ from the piezoelectric materials mentioned above in that manganese (Mn) is further added. Strontium carbonate, calcium carbonate, sodium carbonate, niobium oxide, manganese oxide, and tungsten oxide powders were used as the raw materials. The powders were weighed and dry-mixed in a mortar. The mixture was then placed in an alumina crucible and calcined at 1000° C. to 1180° C. for 2 to 5 hours in air.

Slurries were prepared and molded as in Examples 1 to 8 including performance of magnetic field processing.

The molded bodies were fired in an electric furnace at 1280° C. to 1400° C. for 6 hours in air. The structural and compositional analyses and evaluation of electrical characteristics were carried out as in Examples 1 to 8.

The results of the compositional analysis showed that the obtained piezoelectric materials had a tungsten bronze structure represented by (Sr_(1-x),Ca_(x))_(y)Na₂Nb₁₀O₃₀.

Table 2 shows the composition, F, and d₃₃ of the obtained piezoelectric materials. In the table, a denotes the ratio Sr/Nb on a molar basis, b denotes the ratio Ca/Nb on a molar basis, c denotes the ratio Na/Nb on a molar basis, and x denotes the molar ratio of Ca occupying the A-sites, and y denotes the sum of molar ratios of Sr and Ca occupying the A-sites. In Table 1, “W parts by weight” indicates the ratio of the weight of metallic tungsten to the total weight of the elements, such as Sr, Ca, Na, and Nb, constituting the tungsten bronze structure on an oxide basis; “Mn parts by weight” indicates the ratio of the weight of metallic manganese to the total weight of the elements, such as Sr, Ca, Na, and Nb, constituting the tungsten bronze structure on an oxide basis; F denotes the Lotgering factor F indicating the degree of orientation of (001) plane, and d₃₃ denotes the piezoelectric constant d₃₃.

Comparative Examples 9 to 11

Molded bodies were obtained as in Examples 9 to 11 including the performance of the magnetic field processing except that the tungsten oxide powder was not used.

The molded bodies were sintered as in Examples 9 to 11 to obtain samples of Comparative Examples 9 to 11. The electrical characteristics of the samples of Comparative Examples 9 to 11 were evaluated as in Examples 9 to 11. The composition, F, and d₃₃ of the obtained piezoelectric materials are shown in Table 2.

TABLE 2 Composition W Mn Sr/Nb = Ca/Nb = Na/Nb = parts by parts by d33 a b c x y weight weight F [pC/N] Qm* Example 9 0.360 0.04 0.200 0.100 4.00 1.60 0.10 0.92 119 1060 Example 10 0.360 0.04 0.200 0.100 4.00 1.60 0.60 0.93 122 1188 Example 11 0.360 0.04 0.200 0.100 4.00 1.60 0.90 0.93 101 1401 Comparative 0.360 0.04 0.200 0.100 4.00 0.00 0.10 0.66 78.8 1171 Example 9 Comparative 0.360 0.04 0.200 0.100 4.00 0.00 0.60 0.67 65.6 1264 Example 10 Comparative 0.360 0.04 0.200 0.100 4.00 0.00 0.90 0.67 60.7 1450 Example 11 Qm* Mechanical quality factor

First, Example 9 and Comparative Example 9 are compared. These samples had the same composition except for the W content. Comparison shows that the Lotgering factor F and the piezoelectric constant d₃₃ increase with the W content. The same tendency was observed by comparing Example 10 to Comparative Example 10 and Example 11 to Comparative Example 11. These results suggest that incorporation of W in the material system containing Sr, Ca, Na, Mn, and Nb increases the Lotgering factor F and the piezoelectric constant d₃₃.

Next, Examples 9, 10, and 11 are compared. These samples had the same composition except for the Mn content. Comparison shows that the Lotgering factor F increases with the Mn content. In contrast, the piezoelectric constant d₃₃ does not increase monotonically. These results suggest that incorporation of Mn is effective when the Mn content is within the range of 0.10 to 0.90 parts by weight, but in view of the piezoelectric property, a more preferable range of the Mn content is 0.10 to 0.60 parts by weight.

The mechanical quality factor will now be discussed. In all samples of Examples 9 to 11 and Comparative Examples 9 to 11, the mechanical quality factor is 1000 or higher. In contrast, the mechanical quality factor of Example 6 in which no Mn is contained was 825. This shows that the material system containing Mn has a higher mechanical quality factor than the material system that contains no Mn.

Examples 12 and 13

Strontium carbonate, calcium carbonate, sodium carbonate, potassium nitrate, niobium oxide, and tungsten oxide powders were used as the raw materials. The powders were weighed and dry-mixed in a mortar. The mixture was then placed in an alumina crucible and calcined at 1000° C. to 1180° C. for 2 to 5 hours in air.

Slurries were prepared and molded as in Examples 1 to 8 including performance of magnetic-field processing.

The molded bodies were fired in an electric furnace at 1240° C. to 1380° C. for 6 hours in air. The structural and compositional analyses and evaluation of electrical characteristics were carried out as in Examples 1 to 13.

The results of the compositional analysis showed that the obtained piezoelectric materials had a tungsten bronze structure represented by (Sr_(1-x),Ca_(x))_(y)(Na,K)₂Nb₁₀O₃₀.

Table 3 shows the composition, F, and d₃₃ of the obtained piezoelectric materials. In the table, a denotes the ratio Sr/Nb on a molar basis, b denotes the ratio Ca/Nb on a molar basis, c denotes the ratio Na/Nb on a molar basis, K/Nb denotes the ratio of K to Nb on a molar basis, x denotes the molar ratio of Ca occupying the A-sites, and y denotes the sum of molar ratios of Sr and Ca occupying the A-sites. In Table 1, “W parts by weight” indicates the ratio of the weight of metallic tungsten to the total weight of the elements, such as Sr, Ca, Na, and Nb, constituting the tungsten bronze structure on an oxide basis, F denotes the Lotgering factor F indicating the degree of orientation of (001) plane, and d₃₃ denotes the piezoelectric constant d₃₃.

Comparative Examples 12 and 13

Molded bodies were obtained as in Examples 12 and 13 including performance of the magnetic field processing except that the tungsten oxide powder was not used.

The molded bodies were sintered as in Examples 12 and 13 to obtain samples of Comparative Examples 12 and 13. The electrical characteristics of the samples of Comparative Examples 12 and 13 were evaluated as in Examples 12 and 13. The composition, F, and d₃₃ of the obtained piezoelectric materials are shown in Table 3.

TABLE 3 Composition Sr/Nb = Ca/Nb = Na/Nb = W parts d33 a b c K/Nb x y by weight F [pC/N] Example 12 0.360 0.0400 0.190 0.010 0.100 4.00 1.60 0.91 106 Example 13 0.360 0.0400 0.180 0.020 0.100 4.00 1.60 0.87 103 Comparative 0.360 0.0400 0.190 0.010 0.100 4.00 0.00 0.45 52.1 Example 12 Comparative 0.360 0.0400 0.180 0.020 0.100 4.00 0.00 0.38 48.1 Example 13

According to Tables 1 to 3, all piezoelectric materials of Examples had high F and d₃₃, which suggests that the piezoelectric materials of Examples are superior to the piezoelectric materials of Comparative Examples.

Liquid Discharge Head, Ultrasonic Motor, and Dust Cleaning Device that Use Piezoelectric Materials of Examples 5 and 9

A liquid discharge head shown in FIGS. 3A and 3B, ultrasonic motors shown in FIGS. 4A and 4B, and a dust cleaning device shown in FIGS. 5A and 5B were fabricated using the piezoelectric materials of Example 5 and Example 9. It was confirmed that ink was discharged from the liquid discharge head according to an input electrical signal and that the ultrasonic motors were rotated by application of alternating voltage. A good dust cleaning rate was observed when plastic beads were spread onto the surface of the dust cleaning device by application of alternating voltage.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2010-133290, filed Jun. 10, 2010, and No. 2011-115836, filed May 24, 2011, which are hereby incorporated by reference herein in their entirety.

INDUSTRIAL APPLICABILITY

The present invention provides a piezoelectric material containing no components harmful to environment and having high piezoelectricity. The present invention also provides a piezoelectric element, a liquid discharge head, an ultrasonic motor, and a dust cleaning device that use the piezoelectric material.

REFERENCE SIGNS LIST

-   -   101 piezoelectric element     -   102 individual liquid chamber     -   103 vibrating plate     -   104 liquid chamber wall     -   105 discharge port     -   106 connecting hole     -   107 common liquid chamber     -   108≦buffer layer     -   1011 first electrode     -   1012 piezoelectric material     -   1013 second electrode     -   201 vibrator     -   202 rotor     -   203 output shaft     -   204 vibrator     -   205 rotor     -   206 spring     -   2011 elastic ring     -   2012 piezoelectric element     -   2013 organic adhesive     -   2041 metal elastic body     -   2042 multilayer piezoelectric element     -   310 dust cleaning device     -   330 piezoelectric element     -   320 vibrating plate 

1. A piezoelectric material comprising: a tungsten bronze structure metal oxide that includes metal elements which are strontium, calcium, sodium, and niobium; and tungsten, wherein the metal elements satisfy following conditions on a molar basis: when Sr/Nb=a, 0.320≦a≦0.430, when Ca/Nb=b, 0.008≦b≦0.086, and when Na/Nb=c, 0.180≦c≦0.200; a tungsten content on a metal basis is 0.40 to 3.20 parts by weight relative to 100 parts by weight of the tungsten bronze structure metal oxide; and the tungsten bronze structure metal oxide has a c-axis orientation.
 2. The piezoelectric material according to claim 1, wherein the piezoelectric material has a Lotgering factor F of 0.81 or more and 1.00 or less, where the Lotgering factor F indicates the degree of c-axis orientation determined by X-ray diffraction analysis.
 3. The piezoelectric material according to claim 1, wherein the tungsten bronze structure metal oxide is represented by (Sr_(1-x)Ca_(x))_(y)Na₂Nb₁₀O₃₀ (0.020≦x≦0.200, 4.0≦y≦4.3).
 4. The piezoelectric material according to claim 1, further comprising: manganese, wherein a Mn content on a metal basis is 0.10 to 0.90 parts by weight relative to 100 parts by weight of the tungsten bronze structure metal oxide.
 5. A piezoelectric element comprising: a first electrode; a piezoelectric material; and a second electrode, wherein the piezoelectric material is the piezoelectric material according to claim
 1. 6. A liquid discharge head comprising: the piezoelectric element according to claim
 5. 7. An ultrasonic motor comprising: the piezoelectric element according to claim
 5. 8. A dust cleaning device comprising: the piezoelectric element according to claim
 5. 